Technical Field
The present disclosure relates to linear light-emitting diode (LED) lamps that work with conventional fluorescent lamp fixtures configured to electrically connect either ballasts in double ends or AC mains in a single end.
Description of the Related Art
Solid-state lighting from semiconductor light-emitting diodes (LEDs) has received much attention in general lighting applications today. Because of its potential for more energy savings, better environmental protection (with no hazardous materials used), higher efficiency, smaller size, and longer lifetime than conventional incandescent bulbs and fluorescent tubes, the LED-based solid-state lighting will be a mainstream for general lighting in the near future. Meanwhile, as LED technologies develop with the drive for energy efficiency and clean technologies worldwide, more families and organizations will adopt LED lighting for their illumination applications. In this trend, the potential safety concerns such as risk of electric shock and fire become especially important and need to be well addressed.
In today's retrofit applications of a linear LED tube lamp (LED tube lamp, hereafter in Background section)) to replace an existing fluorescent lamp, consumers may choose either to adopt a ballast-compatible LED tube lamp with an existing ballast used to operate the fluorescent lamp or to employ an AC mains-operable LED tube lamp by removing/bypassing the ballast. Either application has its advantages and disadvantages. In the former case, although the ballast consumes extra power, it is straightforward to replace the fluorescent lamp without rewiring, which consumers have a first impression that it is the best alternative. But the fact is that total cost of ownership for this approach is high regardless of very low initial cost. For example, the ballast-compatible LED tube lamps work only with particular types of ballasts. If the existing ballast is not compatible with the ballast-compatible LED tube lamp, the consumer will have to replace the ballast. Some facilities built long time ago incorporate different types of fixtures, which requires extensive labor for both identifying ballasts and replacing incompatible ones. Moreover, a ballast-compatible LED tube lamp can operate longer than the ballast. When an old ballast fails, a new ballast will be needed to replace in order to keep the ballast-compatible LED tube lamps working. Maintenance will be complicated, sometimes for the lamps and sometimes for the ballasts. The incurred cost will preponderate over the initial cost savings by changeover to the ballast-compatible LED tube lamps for hundreds of fixtures throughout a facility. In addition, replacing a failed ballast requires a certified electrician. The labor costs and long-term maintenance costs will be unacceptable to end users. From energy saving point of view, a ballast constantly draws power, even when the ballast-compatible LED tube lamps are dead or not installed. In this sense, any energy saved while using the ballast-compatible LED tube lamps becomes meaningless with the constant energy use by the ballast. In the long run, the ballast-compatible LED tube lamps are more expensive and less efficient than self-sustaining AC mains-operable LED tube lamps.
On the contrary, an AC mains-operable LED tube lamp does not require a ballast to operate. Before use of the AC mains-operable LED tube lamp, the ballast in a fixture must be removed or bypassed. Removing or bypassing the ballast does not require an electrician and can be replaced by end users. Each AC mains-operable LED tube lamp is self-sustaining. Once installed, the AC mains-operable LED tube lamps will only need to be replaced after 50,000 hours. In view of above advantages and disadvantages of both the ballast-compatible LED tube lamps and the AC mains-operable LED tube lamps, it seems that market needs a most cost-effective solution by using a universal LED tube lamp that can be used with the AC mains and is compatible with a ballast so that LED tube lamp users can save an initial cost by changeover to such an LED tube lamp followed by retrofitting the lamp fixture to be used with the AC mains when the ballast dies.
Ballasts have several different types. However in the US, electronic ballasts are most popular in lamp fixtures because they are more efficient and less expensive than other types of ballasts. Nevertheless, it is better for the ballast-compatible LED tube lamp to be compatible with either electronic ballasts or other types of ballasts.
As mentioned above, a cost-effective solution may be to use a ballast as part of an LED driver to operate a lamp. In some prior art schemes, a switching mode power supply (SMPS) type LED driver is proposed to use with a ballast, but has not been completely accepted due to occasional fires that arise inside the ballast. The cause of these fires has been identified to be a large dc input capacitor in the SMPS type LED driver, which may destroy a capacitor in the ballast due to excessive initial resonant voltage. A conventional SMPS type LED driver for AC mains comprises a Buck converter, which can efficiently convert input voltages of 110˜277 VAC into a DC voltage required to power LEDs in an LED tube lamp. However, the ballast has an output voltage much higher than 277 VAC with a frequency well above 60 Hz. Such a Buck converter is controlled by a control logic, which has several drawbacks that limit its use in ballast applications. First, the control logic has a low operating voltage range which inherently limits the wide range of input voltages that can be used. Second, an over-voltage protection (OVP) function in the control logic starts at a low voltage limited by the low operating voltage. When an input voltage from a ballast exceeds a certain value, OVP functions to stop operation, shutting down the lamp. Third, the Buck converter operates in a continuous conduction mode, in which an input current fails to follow the input voltage, leading to a low power factor with the AC mains and turn-on or other operational failures with the ballast. Fourth, the control logic is solely powered by a voltage built up by an input capacitor with a small capacitance to meet a short start-up requirement. When the input voltage drops to the minimum operating voltage level, the control logic fails to operate and sends no signals to the switch, and the Buck converter stops to function until the input voltage level recovers, resulting in flickering. For an LED tube lamp operating solely with a ballast, the power and current control is basically via an impedance or output voltage control. In the former case, when input frequency changes, the impedance changes, altering an AC current to flow into the driving circuit. A ballast is, in practice, supposed to operate two or more lamps, and its output frequency of the ballast decreases as a load increases, meaning that the total power consumption does not linearly increase as the number of lamps used increases. In the worst case, an LED tube lamp that is designed for a group of three or four lamps in a fixture powered by a ballast may be burned out due to over-rated current flowing into the LED arrays in the lamp if only one of such a lamp is installed in the fixture. For the latter case, the output voltage control approach may work with an electronic ballast but cannot be used in AC mains. In general, conventional LED drivers fail to work with a ballast and to properly operate an LED tube lamp at a regulated power, resulting in unstable lighting output. It goes without saying that the same LED drivers can flawlessly operate the LED tube lamp with the AC mains.
Conventional fluorescent lamp fixtures receive a ballast output voltage from both ends, so called double-ended configuration. When such fixtures are retrofitted double-ended to operate LED tube lamps with the AC mains, a leakage current can flow out of the exposed bi-pin, resulting in an electric shock hazard to an installer. Thus Underwriters Laboratories (UL) require that double shock protection switches be used in the LED tube lamps wired in the double-ended configuration using the AC mains as a power source. However, if the AC mains supply from a single end, i.e. a bi-pin in one end (say, the first end) of the LED tube lamp, with the other end (the second end) electrically isolated from the first end, then the electric shock hazard can be eliminated. One question is: in addition to a single end for AC mains operation, when double ends are also needed for ballast operation, both the first end and the second end are electrically connected to the ballast. How can the LED tube lamps electrically connect to AC mains in a single end and to ballasts in double ends without electric shock hazards? In this patent disclosure, a novel approach will be well addressed.